The French engine maker, who supply the Formula One champions, had a problem-plagued start to the new V6 turbo hybrid era while Mercedes have been dominant since the opener in Australia in March.
Horner was heavily critical of Renault after last month's Austrian Grand Prix, where four times world champion Sebastian Vettel retired and Daniel Ricciardo finished eighth, when he called their engine performance unacceptable.
"We raised our concerns as far back as the end of 2012 with the direction the project was going," Horner told reporters ahead of this weekend's British Grand Prix.
"But designing and building engines is not our core competence... and I don't think any of us could have envisaged that Renault would be as far behind as they have been."
Horner suggested Renault - who supply four teams this season - had erred in not focussing development of the power unit around one car, as Mercedes and Ferrari did.
"When you look at the engine Ferrari has made, the customers have had to adapt their cars accordingly," he said.
"Mercedes likewise, whereas Renault has tried to keep all of their customers happy, which is an admirable thing to do but is not the best way to be competitive."
(Reporting by Alan Baldwin, editing by Josh Reich)
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